In coherent optical communication, it is planned to realize larger capacity by compensation for transmission characteristics and accurate synchronization of timings on a reception side. In various kinds of compensation and timing synchronization, a known signal, for example, a frame synchronization signal (a long-period known pattern signal (LP signal)) inserted in advance for each frame of a reception signal is used. Performance of making capacity larger depends on whether this LP signal can be detected accurately at high speed. Further, in optical communication, transmission characteristics degrade due to polarization dispersion, or the like. Under such circumstances, it is important to accurately detect an LP signal.
In related art, in the case where a signal is received, the signal is correlated with a known signal on the time axis (see, for example, PTL 1). Typically, the signal is correlated for each sampling. As a correlation method, a method in which convolution operation between the signals is performed is common. If a signal pattern matches, the correlation becomes the highest. Note that, in the case where two polarized waves are utilized as in optical communication, the polarized waves are correlated independently from each other.